The present invention relates to a process for the production of vinyl acetate by contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst.
The preparation of supported palladium catalysts for the production of vinyl acetate generally involves impregnating a suitable support with a palladium compound followed by conversion of the palladium compound to substantially metallic palladium.
Methods for the preparation of shell-impregnated catalysts are described, for example, in U.S. Pat. No. 3,822,308, U.S. Pat. No. 4,048,096, U.S. Pat. No. 5,185,308, U.S. Pat. No. 5,332,710, CA 2128162, U.S. Pat. No. 4,087,622, CA 2128154, CA 2128161 and U.S. Pat. No.5,422,329.
Methods for the preparation of non-shell type catalysts are described in, for example, U.S. Pat. No. 3,743,607, GB 1333449, U.S. Pat. No. 3,939,199, U.S. Pat. No. 4,668,819, EP 330853, EP 403950, EP431478 and CA 2071698.
U.S. Pat. No. 5,336,802 describes a method for the pre-treatment of palladium-gold catalysts in which the catalyst is heated in the presence of an oxidising agent such as air at a temperature at least sufficient to partially oxidise the palladium; the oxidising agent is withdrawn and an inert gas such as nitrogen is introduced; the catalyst is then heated again at a temperature up to 500xc2x0 C. in the presence of a reducing agent such as hydrogen or ethylene. The process described therein is illustrated with a xe2x80x9cconventional catalyst containing nominally 1% palladium and 0.5% goldxe2x80x9d.
It is known that the activity for vinyl acetate production of supported palladium catalysts declines with use. If the catalyst""s activity and hence the process productivity declines to a commercially unacceptable level, it is necessary to regenerate and/or replace the catalyst. Deactivation of vinyl acetate catalysts is described by Abel et al. in Chem. Eng. Technol. 17 (1994) 112-118.
Merely increasing the amount of palladium in the catalyst to increase the lifetime of the catalyst presents a problem in that the initial activity of the catalyst may be too high for safe and/or controllable operation on an industrial scale, for example, due to the limited heat removal capacity of the plant.
There remains a need for a process for the preparation of a supported palladium catalyst for use in the production of vinyl acetate which overcomes this problem.
Thus, according to the present invention, there is provided a process for the production of vinyl acetate which process comprises contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps: (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium, and (c) sintering the supported palladium at a temperature of greater than 500xc2x0 C.
The present invention solves the technical problem defined above by sintering the palladium on the support at a temperature of greater than 500xc2x0 C.
Without wishing to be bound by any theory it is believed that this sintering step causes palladium metal particle growth which decreases the initial activity of the catalyst. Thus, catalysts having a high palladium concentration but a commercially acceptable initial activity may be prepared by the process according to the present invention and such catalysts have a longer commercially useful life than conventional catalysts. The sintering step also increases the average pore size of silica supports. The catalysts of the present invention have also been found to be less susceptible to the adverse effects of excess concentration of promoter such as potassium acetate.
The sintering step (c) is preferably performed using a reducing gas, but can be performed in the presence of an oxidising gas or in an inert gas. Suitable reducing gases are hydrogen and carbon monoxide. A suitable oxidising gas is oxygen. These may be diluted with an inert gas. Suitable inert gases for use alone or in conjunction with oxidising or reducing gases are nitrogen, carbon dioxide and helium. Suitable temperatures for the sintering step are from greater than 500 to 1000xc2x0 C. with preferred temperatures being in the range 650-1000xc2x0 C. Preferred times for the sintering step are between 1 and 24 hours. If an oxidising gas is used then the catalyst needs to be subsequently reduced. The catalyst can be purged with an inert gas prior to sintering and during the heat-up period (for safety) and during cool-down (to less than 100xc2x0 C., more preferably to less than 60xc2x0 C.) to prevent any redispersion of the palladium. Any suitable or practicable heat-up and cool-down rates can be used. The sintering step (c) on a commercial scale can be performed in a tower or vessel capable of fulfilling the process conditions outlined above. The catalyst can be agitated by the gas flow during the process. A rotary screw furnace can be used. On the laboratory scale, a horizontal or vertically mounted tube in an electric furnace can be used provided that gas-solid contact is efficient (length/diameter will need to be considered). Pre-heating of the gas stream may be required. The time and temperature of the sintering step are related; the higher the temperature, the shorter the time required. Those skilled in the art will be able to adapt these parameters to fit the scale of operations. Typically the sintering step (c) causes palladium metal particle growth from 3-4 nm in a diameter to 8-15 nm in diameter.
The conversion of the palladium compound to substantially metallic palladium in step (b) may be achieved by a reduction step which can immediately precede the sintering step (c) and by performing the two process steps in the same equipment.
The catalyst preparation process of the present invention may be used for the preparation of uniformly impregnated or shell impregnated catalysts, for use in fluid bed or fixed bed processes for the production of vinyl acetate.
The catalyst preparation process of the present invention may be used to prepare catalysts having high palladium concentrations, for example greater than 0.5% by weight, preferably greater than 1% by weight based upon the total weight of the catalyst. The palladium concentration may be as high as 5% by weight for fluid bed or as high as 10% by weight for fixed bed applications. The initial activity of a supported palladium catalyst having high palladium concentration, if prepared by a conventional process, would be expected to be very high and might even be so high as to be unsafe and/or uncontrollable is used on a commercial scale. However, when prepared by the process of the present invention, the initial activity of the catalyst is reduced compared to that of a conventionally prepared catalyst, whereas the high palladium concentration results in commercially acceptable activity for the extended lifetime of the catalyst.
For the preparation of both shell impregnated and uniformly impregnated catalysts, suitable catalyst supports may comprise porous silica, alumina, silica/alumina, titania, zirconia or carbon, preferably silica. Suitably, the support may have a pore volume from 0.2 to 3.5 ml per gram of support, a surface area of 5 to 800 m2 per gram of support and an apparent bulk density of 0.3 to 1.5 g/ml. For catalysts used in fixed bed processes the support typically has dimensions of 3 to 9 mm. For catalysts used in fixed bed processes the support typically may be spheric, tablet, extrudate, pill shaped or any suitable shape. For catalysts used in fluid bed processes the support typically may have a particle size distribution such that at least 60% of the catalyst particles have a particle diameter of below 200 microns, preferably at least 50% less than 105 microns and no more than 40% of the catalyst particles have a diameter less than 40 microns.
In step (a) the support is preferably impregnated with a palladium compound in a suitable solvent. Suitable solvents may be water, carboxylic acids such as acetic acid, benzene, toluene, alcohols such as methanol or ethanol, nitriles such as acetonitrile or benzonitrile, tetrahydrofuran or chlorinated solvents such as dichloromethane. Preferably, the solvent is water and/or acetic acid. Suitably, the support is impregnated with palladium acetate, sulphate, nitrate, chloride or halogen-containing palladium salts such as H2PdCl4, Na2PdCl4 or K2PdCl4. A preferred water soluble compound is Na2PdCl4. A preferred acetic acid-soluble palladium compound is palladium acetate.
The impregnation of the support may be performed by dipping, immersion or spraying the support in contact with a solution of the palladium compound. The impregnation may be performed in one or more steps or in a continuous process. The support may be contacted with the impregnating palladium solution by tumbling, rotating, swirling or a similar process, to give uniform impregnation. The impregnation is typically performed at ambient temperature. Elevated temperatures may be used for example, with palladium acetate in acetic acid, up to 120xc2x0 C., preferably up to 100xc2x0 C., more preferably up to 60xc2x0 C. Impregnation is performed carefully so as to avoid the break up or attrition of the support. The support can be filled up by the impregnating solution to 5-100% of the pore volume.
In addition to palladium compounds the support may also be impregnated in step (a) with gold, copper and/or nickel compounds, preferably gold, which are converted to the metal along with the palladium in step (b) and are present as mixtures and/or alloys with the palladium in the metallic palladium particles. Suitable gold compounds include gold chloride, tetrachloroauric acid (HAuCl4), NaAuCl4, KAuCl4, dimethyl gold acetate, barium acetoaurate or gold acetate, preferably HAuCl4. These promoters may be used in an amount of 0.1 to 10% by weight of each promoter metal present in the finished catalyst.
In addition to palladium and optional gold, copper and/or nickel the support may also be impregnated at any suitable stage during the preparation process with one or more salts of Group I, Group II, lanthanide or transition metals, preferably of cadmium, barium, potassium, sodium, iron, manganese, nickel, antimony and/or lanthanum, which are present in the finished catalyst as salts, typically acetates. Generally potassium will be present. Suitable salts of these compounds are acetates or chlorides but any soluble salt may be used. These promoters may be used in an amount of 0.1 to 15%, preferably 3 to 9%, by weight of each promoter salt present in the finished catalyst.
The impregnated support may optionally be dried and the impregnation step repeated two or more times if higher palladium or promoter loadings, than the solubility of the salt in the solvent will allow, are required. The drying step may be performed at up to 120xc2x0 C., preferably up to 100xc2x0 C., and most preferably at 60xc2x0 C. The drying step may be performed at ambient temperature and reduced pressure. Air, nitrogen, helium, carbon dioxide or any suitable inert gas may be used in the drying step. The catalyst may be tumbled, rotated or agitated by the gas stream to aid drying.
To prepare shell impregnated catalysts the wet or dry impregnated support is contacted with a base solution with swirling, tumbling, rotation, mixing or the like. The base solution can also be applied by spraying onto the impregnated support during tumbling, rotation, mixing or the like. Bases can be Group I or II hydroxides, carbonates or silicates. Typical examples are sodium hydroxide, sodium metasilicate, potassium hydroxide, potassium metasilicate and barium hydroxide. The base solution can be applied in one or more steps with suitable time delays between applications. The temperature of the precipitation step is usually ambient but can be raised up to 100xc2x0 C. Any solvent can be used in which the base material is soluble, water is preferred. The base should be contacted with the impregnated support for a suitable period so that the metal salts are precipitated in a shell. This usually takes greater than one hour, preferably between 8 to 24 hours. An optimal amount of base will be required for the precipitation and is usually required in excess, commonly this is 1.8 times the notional amount required to generate the hydroxides of the metal salts.
The impregnated support can be washed to remove anion contaminants, for example, nitrates, sulphates and usually halides. For chloride removal, washing with de-ionised water should proceed until a silver nitrate test shows that there is no chloride present. The anion contamination levels should be minimised. Cation contaminants should be minimised; for example to below 0.5 wt %, preferably below 0.2 wt % of sodium in the dried catalyst. Low levels of these contaminants are likely to remain; it is not essential that the levels are absolutely zero. On a commercial scale, batch washing may be used. To speed up the process, warm water may be used. Also, ion exchange solutions (such as potassium acetate) can be used to displace chloride and sodium. Also, the reagents used for the preparation can be selected to avoid the use of chloride and sodium, for example, potassium metasilicate instead of a sodium salt.
In step (b) the palladium compound can be converted to metal before or after the optional washing step above, depending on the reagents used. Liquid reducing agents such as aqueous hydrazine, formaldehyde, sodium formate, methanol or alcohols, preferably aqueous hydrazine can be used. Reduction may also be performed with gases such as carbon monoxide, hydrogen and ethylene. These can be diluted with an inert gas such as nitrogen, carbon dioxide or helium. Typically, the gaseous reduction takes place at elevated temperatures of 100-500xc2x0 C. until the material is reduced. Typically, reduction in the liquid reducing agents takes place and ambient temperatures but temperatures up to 100xc2x0 C. may be used.
After the palladium has been converted to metal it is sintered as herein described. The sintering step (c) may follow on from the step (b) by further heating the catalyst in the reducing gas to greater than 500xc2x0 C. The material may then be impregnated with promoter salts as herebefore described.
The ethylene, acetic acid and oxygen-containing gas may be contacted with the supported palladium catalyst prepared according to the catalyst preparation process of the present invention by methods known in the art. Thus, the reactants may be contacted with the catalyst in a fixed bed or a fluid bed at temperatures in the range 145 to 195xc2x0 C. and pressures in the range of 1 atm to 20 atm. The vinyl acetate product may be recovered by conventional methods known in the art.